1. Technical Field
The present disclosure relates to a biasing circuit for an acoustic transducer, in particular a capacitive microphone of a MEMS (Microelectro-Mechanical System) type, to which the ensuing treatment will make explicit reference, without this implying any loss of generality. The present disclosure further relates to a method for biasing the acoustic transducer.
2. Description of the Related Art
As is known, an acoustic transducer of a capacitive type, for example a MEMS microphone, generally comprises a mobile electrode, in the form of a diaphragm or membrane, set facing a fixed electrode, so as to provide the plates of a variable-capacitance sensing capacitor. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whilst a central portion thereof is free to move or bend in response to the pressure exerted by incident sound waves. The mobile electrode and the fixed electrode form a capacitor, and the bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of said capacitor. In use, the capacitance variation, which is a function of the acoustic signal to be detected, is converted into an electrical signal, which is issued as output signal of the acoustic transducer.
In greater detail, and with reference to FIG. 1, a MEMS capacitive microphone 1, of a known type, comprises a substrate 2 made of semiconductor material, for example silicon; a cavity 3 (generally known as “back-chamber”) is provided in the substrate 2, for example via a chemical etch from the backside. A membrane, or diaphragm, 4 is coupled to the substrate 2 and closes the back-chamber 3 at the top. The membrane 4 is flexible and, in use, deforms as a function of the pressure of the incident sound waves coming from the back-chamber 3. A rigid plate 5 (generally known as “back-plate”) is set over and facing the membrane 4, via the interposition of spacers 6 (for instance made of insulating material, such as silicon oxide). The back-plate 5 constitutes the fixed electrode of a capacitor with variable capacitance, the mobile electrode of which is formed by the membrane 4; the back-plate 5 has a plurality of holes 7, for example with a circular section, designed to enable free circulation of air in the direction of the membrane 4.
Capacitive microphones, and in particular MEMS microphones, receive an appropriate electrical biasing so as to be used as transducers of acoustic signals into electrical signals. In particular, in order to guarantee a level of performance sufficient for the usual applications, the microphones are biased at high voltages (for example 15 V-20 V), typically much higher than those at which a corresponding reading circuit is supplied (logic voltages of, for example, 1.6 V-3 V). For this purpose, it is common to use charge-pump voltage-boosting circuits (generally known as “charge pumps”), made using integrated technology, which are able to generate high voltages starting from reference voltages of lower value.
A common circuit configuration (illustrated in FIG. 2) envisages that a charge-pump stage, illustrated schematically and designated as a whole by 8, is directly connected to a first terminal N1 (constituted, for example, by the back-plate 5) of the MEMS microphone 1 (represented schematically by the equivalent circuit of a variable-capacitance capacitor), so as to supply biasing voltages of high value. A second terminal N2 (for example, constituted by the membrane 4) of the MEMS microphone 1 is instead connected to the high-impedance input of a reading circuit (also defined as “front-end circuit”), in the figure represented schematically as an amplifier stage 9 (the high impedance of which is in turn represented schematically by an input resistor 10 having a resistance typically comprised between 100 GΩ and 100 TΩ, connected between the second terminal N2 and a reference-voltage node, e.g., coinciding, as in the case illustrated, with the ground of the biasing circuit).
This circuit arrangement is, however, considerably limited by the reduced signal-to-noise ratio since, during normal operation, both a possible “ripple” at the output of the charge-pump stage 8 and the noise generated by the same charge pump add, without any attenuation, to the electrical signal generated by the MEMS microphone 1 as a function of the detected acoustic signal.
To overcome the above limitation, an alternative circuit arrangement has been proposed (shown in FIG. 3), in which a low-pass filter 12, in RC configuration, is set between the output of the charge-pump stage 8 and the first terminal N1 of the MEMS microphone 1 so as to appropriately attenuate both the ripple and the noise at output from the charge-pump stage. In particular, the low-pass filter 12 is made by a filter resistor 13, connected between the output of the charge-pump stage 8 and the first terminal N1 of the MEMS microphone 1, and by a filter capacitor 14, connected between the same first terminal N1 and a ground terminal of the biasing circuit.
It has, however, been shown that, in order for the low-pass filtering action to be effective and be able to obtain an appropriate biasing of the MEMS microphone 1, the low-pass filter 12 should have a pole at a frequency equal to or preferably lower than 1 Hz. For this purpose, the filter resistor 13 should have a resistance of extremely high value, for example comprised between 100 G Ω and 100 T Ω.
Given that, as it is known, it is not possible, in integrated-circuit technology, to obtain resistors with such high resistances, the use of non-linear devices capable of providing the high values of resistance has been proposed. For example, for this purpose it has been proposed the use of a pair of diodes in anti-parallel configuration, providing a sufficiently high resistance when a voltage drop of contained value is set across them (the value depending on the technology, for example being less than 100 mV).
As illustrated in FIG. 4, both the filter resistor 13 and the input resistor 10 can hence be provided by a respective pair of diodes in anti-parallel configuration.
In particular, the filter resistor 13 is provided by a first diode 13a, with its anode connected to the output of the charge-pump stage 8 and its cathode connected to the first terminal N1, and by a second diode 13b, with its anode connected to the first terminal N1 and its cathode connected to the output of the charge-pump stage 8. The input resistor 10 is provided by a respective first diode 10a, with its cathode connected to the second terminal N2 and its anode connected to the reference voltage, designated here by Vref, and by a respective second diode 10b, with its cathode connected to the reference voltage Vref and its anode connected to the second terminal N2.
The main problem of such a circuit architecture is represented by the long start-up time of the biasing circuit in general and of the low-pass filter 12 in particular, principally due to the presence of the pair of diodes connected in anti-parallel configuration and to the high resistance provided thereby. The settling time of such a configuration can easily last minutes or even hours. Before the end of this settling time, i.e., for the entire start-up time of the circuit, proper operation of the low-pass filter 12 cannot be guaranteed, nor likewise can proper biasing of the terminals N1, N2 of the MEMS microphone 1 be guaranteed. Hence, during the start-up time, inevitably even considerable variations occur in the sensitivity associated to the MEMS microphone 1, as detected by the reading circuit.
In particular, as illustrated in FIG. 5, the voltage of the first terminal N1 (designated by V1) moves slowly towards the desired biasing voltage value, equal to the pump voltage supplied by the charge-pump stage 8 (designated as Vcp), whilst the voltage of the second terminal N2 (designated as V2) moves slowly towards the value of the reference voltage Vref (in FIG. 5 the voltage drop across the pair of diodes in anti-parallel configuration 10a, 10b is designated as Vd). Only at the end of the long start-up time do the voltages of the first and second terminals N1, N2 stabilize at the desired biasing voltages (steady-state situation).
Clearly, such long delay times are not acceptable in the common situations of use of the MEMS microphone 1 when, instead, it is necessary to guarantee the nominal performance (and, in particular, a substantially constant sensitivity) with extremely brief delays both in turning-on of the electronic device incorporating the MEMS microphone and upon return from a so-called “power-down” condition (during which the device is partially turned off to ensure an energy-saving condition).
As a possible solution to this further problem (as illustrated in FIG. 6), the use of a highpass-filter stage 15 has also been proposed, connected in series to the output of the amplifier stage 9 (which constitutes the first signal-processing stage of the reading circuit associated to the MEMS microphone 1) so as to “mask” the long settling time of the biasing circuit. However, also this solution is not free from drawbacks, in particular as regards the greater occupation of area, the circuit complexity of the resulting reading interface, and the possible distortions introduced by the further filtering stage.